1. Field of the invention
The present invention relates to a molded plastic package for semiconductor devices of the so-called Single-In-Line (SIL) type to be associated with an external heat sink.
2. Description of the prior art
There are many known molded plastic packages for medium power semiconductor devices of the so-called Single-In-Line type which normally comprise a metal backing plate, often only partially embedded in the molded encapsulating plastic body and to which the semiconductor die encapsulated within the package is thermally connected. The die may be bonded directly on an internal surface of the metal backing plate, the opposite face of which remains exposed at least partially on a major surface of the encapsulating plastic body, which major surface forms a dihedral angle width and is normally perpendicular to a minor surface of the plastic body through which emerge the pins of the device. The generally polyhedric package is most commonly a flat parallelepiped, the major surface of which, where the surface of said partially embedded metal backing plate remains coplanarily exposed, is normally in contact with a flat coupling surface of an external heat sink, which is commonly a finned aluminum body. In order to ensure good heat conduction between the external heat sink and the device or the devices electrically connected through their plurality of pins to a printed circuit card, it is necessary to provide for mechanical fastening means between the devices and the external heat sink. These fasteners are generally screws, the stem of which passes through holes formed through the thickness of the flat package or of said metal backing plate onto which the encapsulating resin is molded.
Often the assembly of integrated devices and other circuit components on printed circuit cards and the execution of the tin soldering which is commonly carried out by the socalled wave-bath technique utilizes highly automated handling techniques of the various pieces which are incompatible with the presence of heavy and cumbersome metal heat sinks on the printed circuit cards. This makes it necessary to process the cards without the heat sinks which must be mounted later in a coupling relationship with generally a plurality of SIL devices which have been purposely mounted in a line on the card. The fact that the soldering of the pins of the devices on the card has already taken place and that departures from a perfect alignment between the fastening holes of the devices and the respective threaded holes pre-formed in the heat sink are inevitable, makes the assembly of the heat sink and the fastening thereof to the single devices difficult. These operations frequently must be carried out manually and the assembler must often cope with mismatchings among respective assembly holes. In other words the critical aspect is represented by the fact that the external heat sink has a plurality of threaded holes for the fastening screws of the devices which must be presented in a perfect alignment to the respective fastening holes of the devices which have already been soldered onto the printed circuit card. Moreover the various fastening screws must be singularly inserted and screwed in.